Collective behaviour of herring during spawning

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Abstract

The thesis focuses on the collective behaviour of Norwegian spring spawning
herring (NSS-herring; Clupea harengus L.) in the specific ecological context of
reproduction. The major part of the work is based on field research using
quantitative echo sounders and sonar in combination with biological sampling. In
addition, an individual based schooling model is applied to investigate underlying
mechanisms of the collective formations and dynamics. All studies are founded in
a classical mechanistic ecological approach interpreting behaviours as optimal
evolutionary strategies given an individual’s physiological and cognitive
constraints, internal state and the specific ecological context.
Two of the works are case-studies from historical spawning grounds of NSSherring
well-known to fishermen and scientists; the shallow (30-40 m) banks off
Karmøy (Paper 1) receive a small proportion of the spawning stock, whereas deep
(80-250 m) areas off Møre (Paper 2) comprise the main spawning grounds. In
both works the areas are covered through repeated acoustic surveys in
combination with sampling of herring and predators throughout the 24-h-cycle
over several days. Fisheries scientists apply similar acoustic surveying when
making abundance estimates of herring that serve as basis for advices of catch
quotas. A major challenge during such surveying is the fact that herring tend to
avoid the approaching research vessel, rendering them unavailable to detection.
How this potential source of error operates during spawning is given the focus in
one of the works (Paper 3). In this study we use a simple experimental design
where a standard research vessel passes a presumably neutral stationary vessel
that records the herring reaction. In the fourth work we apply a rule-based
simulation model with high temporal resolution able to recreate life-like collective
behaviours (Paper 4). The model is used to explore how collective dynamics and
formations are affected when varying the amount of herring present and their
degree of motivational synchronisation. In the final work (Paper 5) we investigate
11 years of acoustic data from herring spawning surveys along the Norwegian
coast. We frequently find evidence of vertical hourglass formations, and based on
knowledge gained from previous studies and the simulation model we formulate
and test out a concrete hypothesis; are these formations founded in conflicting
individual motivation for spawning?
An overall evaluation of our field results strongly supports that herring behaviour
during spawning reflects a compromise between survival and reproduction. We
find that herring spawn in highly synchronous waves where individuals aggregate
(Paper 2) and move together (Paper 1) in their quest for a successful spawning.
This strategy is likely to be advantageous for survival since a synchronous
emergence in high numbers will decrease an individual’s risk of being targeted by
a predator (dilution effect). A similar advantage is gained through spawning in the
dark hours when predators are less active. Shoals at night-time were observed to
have a stable loose packing density, whereas a variable, but generally high
packing density at daytime corresponds well with a presumed higher frequency of
predator attacks, since it is known that shoals become more densely packed under
attack. At the deep spawning grounds, large proportions of the herring became
scattered in pelagic layers at night-time. Interestingly, these layers were
positioned according to the thermocline in a way that indicates active use of the vertical temperature profile to fine-tune maturation rate in the days prior to
spawning. The shallow spawning grounds had a different dynamic and such layers
were not observed. In these areas, the vast majority of herring were in layers at
the bottom at night-time. In this situation, herring did not react to the research
vessel (Paper 3), probably because the focus on the spawning activity increases
their reaction threshold. At both spawning grounds (Papers 1 and 2), there were
substantial vertical dynamics that corresponded well with the results from the
schooling model. In the model simulations, shoal height is dependent on the
difference in individual motivation for spawning. With strong conflicts in
motivation, shoals tend to split in the same way as we observed during spawning
in the field (Paper 2), whereas with intermediate conflicts shoals maintain
cohesion but become vertically extended like we observed during pre-spawning.
A special case of the latter is the vertical hourglass formation that emerges in the
model assorted according to maturation state. Similar assortment was observed in
the hourglass formations in the wild, where spawners dominated in the lower
parts relative to pre- and post-spawners, strongly indicating that motivational
conflict is the driving force of the observed formation. Whether such formations
are purely emergent from decisions made from local stimuli or implying
communication between the two shoal parts is an interesting question open to
future investigations.